DESCRIPTION (Adapted from applicants' abstract) The long term goal of the proposed studies is to understand the structure/function relationships of the two major isoforms of the myosin heavy chain (MyHC). The MYHC is the largest protein in the cardiac sarcomere and, in vertebrates, exists predominantly in two isoforms, V1 and V3, encoded by a-MyHC and B-MYHC, respectively. The two isoforms have different expression patterns based on species, developmental stage, and disease, additionally, they have different intrinsic ATPase activities. In humans, mutations in the more abundant B-MyHC isoform have been linked to familial hypertrophic cardiomyopathy. Most of the known mutations are clustered in the globular head of the protein, which contains the sites of actin and essential light chain binding, the site of ATPase activity, and the region connecting two reactive cysteines. In order to define the role of isoform specificity, transgenic and targeted ("knockout") mice will be used for the following Specific Aims: (1) Overexpression of the mouse B-MyHC isoform (V3) on the wild type a-MyHC+/+ (V1) background to generate mice with varying V1/V3 ratios, and (2) Complete replacement of the V1 isoform with V3 by expressing the B-MyHC isoform on the a-MyHC null ("knockout") background. Thorough analysis of these animals from the molecular to whole organ level will highlight the role of isoform specific ATPase activity in modulating the complex protein interactions of the cardiac sarcomere which underlie cardiac function. (End of Abstract)